Better error message for illegal yield in classic mode

[bnoordhuis]

async function g() { // note: not a generator function
    yield 42;        // same error message without the semicolon
}

Raises unexpected 'yield' keyword in ESM mode but expecting ';' in classic mode.

It's not strictly wrong because yield is a valid identifier in classic mode (you get the same error with yolo 42) but it's unhelpful.